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This is not too difficult, once you get the hang of it.

Converting a recipe simply involves removing the natural leaven, or yeast, from the recipe and then substituting the other form of leaven. It is a two step process. There is no workable single step conversion between the two forms of leavening bread.

I prefer the term natural leaven to sourdough starter and that term is used here. A poolish converts to a yeast recipe in exactly the same way as a natural leaven recipe converts.

All percentages here are bakers’ percent which is a percentage of the total flour being used. If 100g of flour is being used then the water at 65% hydration is 65g. Bakers percentages are always calculated as if the weight of the flour is 100% and all other ingredient weights are a percentage of that.

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**To convert a recipe using yeast to a natural leaven (Sourdough recipe)**

Leave the yeast packet in the fridge. Calculate the amount of leaven you want to use and deduct the flour and water in it from the flour and water in the recipe.

A range of 10% - 30% total weight of leaven is typically used.

So, if using 20% natural leaven in a loaf with 500g of flour the leaven is going to weigh 200g. If the leaven is half water and half flour then it will have 100g of water and 100g of flour. Subtract 100g from both the weight of water and the weight of flour in the recipe.

**Converting a natural leaven recipe to one using yeast.**

To convert a natural leaven recipe to one using yeast.

First work out how much flour and water is in the leaven. Add these back into the recipe.

Secondly calculate the amount of yeast you need and use that.

**How much Yeast to use**

Some manufacturers don’t recommend yeast dosages for their products. There is little difference between the brands and many quite a few brands are actually re-labelled.

These days fresh yeast, active yeast and instant yeast are all from the same yeast fermentation Vats. They are merely finished using different processes. There is no flavour advantage in using fresh yeast. Instant yeast has better keeping qualities and there is a higher percentage of live yeast in it compared to fresh and active yeast. Dead yeast can diminish loaf volume.

Use fresh yeast at 2% of the total flour weight in the recipe.

Use active yeast at 0.93% (or 1%) of the total flour weight. That’s 2 ¼ level teaspoons for 500g of flour.

Use instant yeast at 0.66% - 1% of the total weight of the flour in the recipe. 0.66% is a level teaspoon, to 500g of four. 1% instant yeast can be used to ferment your dough faster, but remember fast fermentation means less flavour brewed into your loaf.

Quite a few recipes online and from flour manufacturers, use much higher levels of yeast than the figures given above. I can only assume that they want their recipe to be fool proof. Using higher levels of yeast makes the bread stale more quickly.

**What to do when you don’t know how much flour and water is in the leaven.**

Hopefully the recipe will tell you. Most YouTube bakers use half water to half flour and quote the leaven as a total weight. So 100g of leaven will be 50g of flour and 50g of water. Simply add these amounts to the flour and water in the recipe.

Italian Biga’s are typically 56% to 65% water. If you don’t know the level of water in the biga 60% hydration is a good guess. So, divide the weight of the biga by (60 + 100) and then multiply it by 0.6 and that is the weight of the water in it. The remainder is flour. Again, add these weights to the recipe.

Occasionally we come across 80% and 120% hydration natural leavens. The maths is the same for the biga above.

Calculate the weight of water in a leaven in the same way as shown above for the Biga. For an 80% hydration leaven the amount of water in it = the weight of the leaven, divided by 180 (100 + 80) and multiply the result by 80. The remaining weight is flour. 200g of 80% hydration leaven divided by 180 = 1.1.

1.11 x 80 = 89g of water.

*Tip: If you don’t know the amounts of flour and water in the leaven simply assume it is half flour and half water. After mixing the recipe stand the dough for 30 minutes to let the flour hydrate and add a little more water to get a good dough consistency. Typically, you might be adding more water at a mere 5% of the total flour weight. In a dough using 500g of flour that is 25g of water.*

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